After a wet night or a long drive in heavy rain the car misfires, hesitates, or blows a puff of white from the tailpipe, and it clears once the engine is warm and dry. The charge pipe is then found to have water in it. The core is doing its job; the problem is that something is delivering water to its face.
A charge cooler has no way to tell water from air. It sits in the airstream, it cools whatever passes through, and it will pass a slug of water into the engine with the same indifference it shows to a hot charge. The question is always the source, not the core.
An intercooler lives in the highest-pressure, most exposed part of the airstream, usually ahead of the radiator and often behind a bumper vent that sits close to the road. That is exactly where water is thrown, and the pressure that feeds the core is the same pressure that drives spray through it.
The usual source is not rain falling into the engine bay. It is spray lifted by the front tyres, thrown forward and sideways into the bumper opening. A missing or trimmed undertray, an enlarged bumper vent, or a duct that ends low and open makes it dramatically worse, and the fault then appears only in weather.
Humid air carries water vapour, and the core cools the charge, so some of that vapour condenses on the cold surfaces. In a healthy system this is a small amount that is carried through as vapour or drains away, and it does not pool. A little clear water at a low point after a cold, damp night is not a fault.
Condensation water is clear and odourless and appears after a cold start in damp weather rather than after a wet drive. Splash water is also clear but arrives in quantity, often carrying road grit, and it correlates with the weather rather than with the warm-up cycle. That timing is the whole distinction.
If the system is air-to-water, water in the charge tract may not be rain at all. A failed internal core, a cracked end tank weld, or a hose leaking inside the housing will put coolant into the air path. That is a much more serious fault, because the level in the reservoir falls and the engine is ingesting coolant.
Smell and colour separate them. Coolant has a sweet smell and usually a colour, and the level in the aluminium water tank drops without any external leak. Rain water is clear and has no smell. If the fluid is coolant, stop driving and pressure test the water circuit before anything else.
Inside the core, water occupies volume that air should be using, and it does not compress. A quantity of water reaching a cylinder can raise pressure beyond what the engine can contain. Even in small amounts it displaces air, so the mixture goes rich, the engine misfires, and the charge temperature reading can behave oddly.
Water also sits in the low rows and the end tank, so the effect can persist after the weather clears. That is why a car that misfires in rain and then runs clean can still have water in the system days later, and why a drain, where one exists, matters more than it looks.
Look for every path by which water can reach the core and close it off. Restore the undertray and any splash shields, check that the bumper duct does not aim at the core face from a low, wet position, and confirm the core has a way for what does enter to leave rather than pool in a corner.
Then inspect the core for a wet pattern on the face and check the lowest joint for water. Where the core is mounted low and exposed, end tanks that drain and a feed that takes air from higher up will do more for the problem than any change in core size, and the routing is usually the cheaper thing to change first.
